The degree of resistance offered by the boiler passes to the flow of the gases is an important consideration in determining the required amount of draft. These gases must exist at a speed sufficient to prevent them from backing up into the combustion chamber and robbing the fire of necessary oxygen.

The total area of the grate, the type of coal burned (e.g., bituminous, semibituminous), the size of the coal, and the thickness of the coal bed all affect the amount of draft required for proper combustion.

Insufficient draft usually results in the accumulation of excess ashes in the ashpit. Moreover, it necessitates additional attention to the fire including more frequent cleaning. These and other aspects of improper firing contribute to fuel waste and higher operating costs.